1. Field of the Invention
This invention relates generally to discrete analog signal processing and more particularly to such systems as comprised of charge transfer devices (CTD's) for sampling and providing discrete delays to an input signal and an array of MNOS memory devices capable of being programmed to multiply the outputs of the CTD device in accordance with a defined function. Such CTD and MNOS devices are particularly adapted to be implemented by large-scale integration (LSI) techniques.
2. Description of the Prior Art
In the prior art, digital signal processing (DSP) has been utilized in many applications in view of the low cost of the integrated circuits that are available to perform many functions in digital fashion. As compared with analog techniques, DSP has been preferred in that its use of digital implementation has been considered to be preferable in view of cost, weight, flexibility and accuracy considerations. By contrast, the only present advantage of analog techniques is its lower power consumption, which advantage may gradually disappear as technology advances. A further technique known as discrete analog signal processing (DASP) provides an alternative to the aforementioned methods of signal processing and is implemented by sampling at regular intervals an analog signal to provide a series of analog signals or bits, each of which may be operated upon one-at-a-time and have an amplitude containing information or data corresponding to M digital bits, where one analog bit equals M digital bits. Thus, the number of operations performed in DASP is significantly reduced, thereby requiring a corresponding less number of elements to carry out the desired processing.
In DASP, the input analog signal is sampled at a predetermined, constant or frequency f.sub.c, which samples are stored, transferred and operated upon by analog means. In DSP, digital or quantized samples are handled with binary logic. In a similar manner, it has been suggested that charge coupled devices (CCD's) are uniquely capable of effecting DASP in that they are uniquely capable of sampling and non-destructively storing at each of their outputs the delayed signals to be processed in an analog fashion. Such DASP systems incorporating CTD's, retain the capability of precise transport delays, of particular interest in coherent processing. The dynamic range where one bit of resolution in DSP is equivalent to 6dB dynamic range in the analog signal. Experiments have shown that a signal charge analog packet can be shifted through a typical CCD nearly unattenuated, limited by the size of the holding wells and the minimum detectable output signal.
As more fully described in an article entitled, "Charge Coupled Semiconductor Devices" appearing in Bell System Technical Journal, April 1970, by W. S. Boyle and G. E. Smith, CCD's sample an analog input signal to provide a series of analog bits to be stored in potential wells created at the surface of a semiconductor and transported along the surface by timing or phase signals. More particularly, these charges constitute minority carriers stored at the silicon-silicon dioxide interface of MNOS non-memory capacitors and are transferred from capacitor or well to capacitor or well on the same substrate by manipulating the voltages applied across the capacitor.
As described in the articles entitled, "Transversal Filtering Using Charge Coupled Devices" by D. D. Buss, D. R. Collins, W. H. Bailey and C. R. Reeves, published in IEEE Journal of Solid State Circuits, SC-8, pp. 138-46, April, 1974, and "A Surface Charged Correlator For Signal Processing" by J. J. Tiemann, R. D. Baertsch and W. E. Engeler, as presented at the CCD Applications Conference, September 1973, CCD's may be adapted for analog signal processing and in particular, for use as transversal filters wherein the CCD's effect a serial transfer of charge packets along a linear path. At each stage, the amount of charge may be non-destructively measured at its corresponding tap. To form a transversal filter, the tap outputs are approximately weighed and summed. As suggested in the noted article, a tap weight may be provided by dividing each stage of the CCD into two portions and sensing only charge on one of them through the capacitor electrode. Summation of the charge residing on all the selected taps is performed by connecting all of the capacitor electrodes to a common drive circuit and monitoring the drive current required to charge or to discharge them. Thus, the multiplicative tap weights are determined by the location of cuts in the capacitor electrode plates and the addition is performed by strapping the capacitor electrodes together. However, the weights provided by the described electrode configuration are fixed and there is no suggestion as to how tap error may be compensated. Further, the analog signals are mixed with clock currents and the dynamic range of such a system is restricted.
A cross-correlator performs similar functions as a transversal filter, differing in that the tap weights are set in accordance with a second input signal. Thus, a cross-correlator can implement all of the possible transversal filter functions and may be considered as a truly general-purpose signal processing device. However, a cross-correlator requires that the tap weights be programmable in accordance with an input signal. The above-described method for achieving tap weights by cutting the capacitor plate results in a fixed weighting unsuitable for cross-correlator devices. Thus, the noted article by Tiemann et al. concludes that "there is no convenient way of obtaining an electrically variable multiplicative fraction of the surface charge in the storage reservoir", thus eliminating use of the described system for such purpose.
Further, there has been suggested that the use of MNOS transistors of different conductances to provide the desired weighting at the non-destructive taps of the CCD device. Although this approach avoids the mixing of analog and digital transients, the weights provided by the transistors are still fixed, thus limiting the flexibility of the resulting system and in particular making it unsuitable for use as a cross-correlator as noted above.